Probability Calculator Formula

Understand the math behind the probability calculator. Each variable explained with a worked example.

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Formulas Used

Probability

probability = favorable / total

Probability Pct

probability_pct = (favorable / total) * 100

Odds For

odds_for = favorable / (total - favorable)

Odds Against

odds_against = (total - favorable) / favorable

Variables

VariableDescriptionDefault
favorableFavorable Outcomes3
totalTotal Outcomes10

How It Works

How to Calculate Probability

Formula

P(event) = Favorable Outcomes / Total Outcomes

Probability ranges from 0 (impossible) to 1 (certain).

Odds

  • Odds in favor = favorable / unfavorable
  • Odds against = unfavorable / favorable

    • Worked Example

    A bag has 10 marbles: 3 red, 7 blue. What is the probability of drawing a red marble?

    favorable = 3total = 10
    1. 01P(red) = 3 / 10 = 0.3
    2. 02As a percentage: 30%
    3. 03Odds in favor: 3 / 7 ≈ 0.4286
    4. 04Odds against: 7 / 3 ≈ 2.3333

    Frequently Asked Questions

    What is probability?

    Probability is a measure of how likely an event is to occur. It is calculated as the number of favorable outcomes divided by the total number of possible outcomes.

    What is the difference between probability and odds?

    Probability is favorable/total (ranges 0 to 1). Odds are favorable/unfavorable (can be any positive number). Probability of 0.75 means odds of 3:1 in favor.

    Learn More

    Guide

    How to Calculate Probability - Complete Guide

    Learn the fundamentals of probability including basic probability, compound events, conditional probability, Bayes' theorem, and expected value with worked examples.

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